In text I/O, optimize scanning for new lines with 1-byte unicode chars

Modules/_io/textio.c

@@ -365,6 +365,9 @@ _PyIncrementalNewlineDecoder_decode(PyObject *_self,
             */
             if (seennl == 0 &&
                 memchr(in_str, '\n', kind * len) != NULL) {
+                if (kind == PyUnicode_1BYTE_KIND)
+                    seennl |= SEEN_LF;
+                else {
                     Py_ssize_t i = 0;
                     for (;;) {
                         Py_UCS4 c;
@@ -380,6 +383,7 @@ _PyIncrementalNewlineDecoder_decode(PyObject *_self,
                             break;
                     }
                 }
+            }
             /* Finished: we have scanned for newlines, and none of them
                need translating */
         }
@@ -1597,6 +1601,10 @@ textiowrapper_read(textio *self, PyObject *args)
 static char *
 find_control_char(int kind, char *s, char *end, Py_UCS4 ch)
 {
+    if (kind == PyUnicode_1BYTE_KIND) {
+        assert(ch < 256);
+        return (char *) memchr((void *) s, (char) ch, end - s);
+    }
     for (;;) {
         while (PyUnicode_READ(kind, s, 0) > ch)
             s += kind;